Iowa darter

Etheostoma exile

By Tom Dickson

This story is featured in Montana Outdoors May–June 2009

The walleye and sauger get plenty of attention in central and eastern Montana, but you never hear much about their colorful little cousin, the Iowa darter.
Darters are zippy little fish that swim along the bottom of streams or lake shallows in short bursts. These tiny members of the Percidae family, also related to the yellow perch, are found only in North America. Most of the continent’s roughly 140 darter species live in the eastern United States, particularly Appalachia. None are native west of the Rocky Mountains. The westernmost range of any darter is that of the Iowa darter, which is found as far west as Alberta and north-central Montana.

Identification The Iowa darter is sometimes mistaken for a minnow, but it is unrelated to shiners, chubs, dace, or other minnow species. It has a slender body, which explains the second part of its scientific name, exile, Latin for “slim.” (The first half, Etheostoma, comes from two Greek words meaning “straining mouth,” for reasons unknown). Like other percids, the Iowa darter has a two-part dorsal fin, spiny in the front and soft-rayed in the back. Like those of so many darter species, the spawning male Iowa is brightly colored. His body is olive green with roughly ten bluish green saddle bands across the back and sides, interspersed with rust red splotches. The front dorsal fin is brilliantly colored in blue-green with blood red marks between each spine. The massive pectoral fins are fan shaped, the tail is squarish, and the head is conical. A dark blue tear-shaped mark extends down from each eye. Iowa darters average only 2 inches long, making them one of Montana’s smallest fish. (The photograph above is actual size.)

Behavior Most other fish can float in the water column, kept buoyant by a swim bladder, or air bladder. Lacking this feature, the Iowa darter sinks to the stream or lake bottom. To move around, it makes short dashes of astonishing speed (hence the name), often faster than the human eye can follow. The darter can zip along stream and lake substrate looking for food using less energy than other fish, which would keep floating up off
the bottom.

Food The Iowa darter eats copepods, water fleas, and midge and mayfly larvae it finds in underwater vegetation. You’d think such a small fish living in shallow water would be prime prey for herons, kingfishers, or predator fish. But I have never found a report of Iowa darters in the stomach of any fish-eating animal. Maybe they are just too darn quick to catch.

Reproduction No one has studied the mating habits of Iowa darters in Montana. But in other states where the fish is common, biologists have learned that the fish spawns in just a few inches of water among underwater vegetation or tree roots. After establishing a territory, a male chases any intruding males away. When a female shows up, he swims around and around her until she stops over the vegetation or roots he has claimed. He gets on top and holds her in place, his pelvic fins over her dorsal fin. She vibrates to release eggs, which stick to the vegetation, as he releases milt.

Range In Montana Iowa darters inhabit tributaries of the Missouri River below Fort Peck Dam, the Little Missouri River basin in the southeastern corner of the state, and throughout the Milk River system as far west as Fresno Dam.

Seeing one for yourself The best way to find an Iowa darter is to wade the shallows of the streams and rivers listed above in April and May while looking closely in the vegetation for splashes of orange or blue that indicate the spawning males. Capture one for observation using a small-mesh seining net. Don’t be discouraged if you don’t nab one of these lightning-fast rascals on the first attempt. One late-19th-century ichthyologist wrote that it would be easier to grab a weasel than it would be to catch a darter.